ruff

ce8b744f - [ty] Only calculate information for unresolved-reference subdiagnostic if we know we'll emit the diagnostic (#18465)

[ty] Only calculate information for unresolved-reference subdiagnostic if we know we'll emit the diagnostic (#18465) ## Summary This optimizes some of the logic added in https://github.com/astral-sh/ruff/pull/18444. In general, we only calculate information for subdiagnostics if we know we'll actually emit the diagnostic. The check to see whether we'll emit the diagnostic is work we'll definitely have to do whereas the the work to gather information for a subdiagnostic isn't work we necessarily have to do if the diagnostic isn't going to be emitted at all. This PR makes us lazier about gathering the information we need for the subdiagnostic, and moves all the subdiagnostic logic into one function rather than having some `unresolved-reference` subdiagnostic logic in `infer.rs` and some in `diagnostic.rs`. ## Test Plan `cargo test -p ty_python_semantic`